i18n: check equality of initial indentation between msgid and msgstr Document generation by runrst in "doc" directory may succeed silently, even though initial indentation is different between msgid and msgstr: for example, it may be unexpected or missing indentation. This patch adds the checker to check equality of initial indentation between msgid and msgstr. This checker is categorized as "warning" level, because problem detected by this is not so serious for usual Mercurial usage.